The present invention relates to an optical waveform shaping device which performs waveform equalization and timing synchronization for a high-speed digital fiber optic communication by the use of an optical pulse signal having a pulse duty factor within 1 with respect to a time slot.
With the recent progress of optical amplification techniques, the fiber optic communication technology permits realization of an ultra-long distance transmission across the Pacific Ocean, for instance, without using any conventional regenerative repeaters. With this linear optical amplifying-repeating transmission, however, as the transmission rate increases, the transmitted waveform undergoes gradual deterioration which is caused by the wavelength dispersion characteristic and nonlinear optical effect of the optical fiber used, imposing limitations on the ultra-high-speed long distance transmission. In recent years an optical soliton communication system has been in the limelight as a system which surmounts the limitations on the speeding up of transmission owing to the wavelength dispersion characteristic and nonlinear optical effect. The optical soliton system is one that makes positive use of the wavelength dispersion characteristic and nonlinear optical effect of the optical fiber which are contributing factors to the deterioration of the transmission characteristic of the above-mentioned prior art system and that transmits short optical pulses intact while balancing their broadening by the wavelength dispersion in the optical fiber and their compression by the nonlinear optical effect. A time multiplex and a wavelength multiplex system are also relatively easy to implement and are suitable for high-speed, large-capacity transmission.
In case of implementing the optical soliton communication system of large optical amplifying repeater spacing by selecting the amplifier gain large in a transpacific or similar ultra-long distance optical amplification system, a noise component of light spontaneously emitted from each optical amplifier increases and as the distance of transmission increases, such noise components are accumulated, causing a decrease in a signal to noise ratio of the whole system and hence deteriorating receiving characteristics. The main factor in deterioration of the receiving characteristics is a timing jitter resulting from a random velocity modulation of optical soliton pulses which is caused by the interaction of the accumulated noise components and the nonlinear optical effect of the optical fiber. If the pulse to be received, accompanied by the timing jitter, does not arrive within the time slot of a signal, an error will be induced. To avoid this, it is necessary that the gain of the amplifier be low to suppress the noise, and the spacing of the optically amplifying repeaters becomes relatively small.
On the other hand, there has been proposed a method which prevents an increase of such a timing jitter by retiming and waveform shaping of the transmitted signal by an optical modulator after allowing the passage of the signal through a certain number of optical amplifying repeaters (Publication 1; M. Nakazawa et al., "10 Gbit/s soliton data transmission over one million kilometers," Electronics Letters, Vol. 27, pp. 1270-1272, July 1991). According to publication 1, a 510-km loop is prepared which is formed by optical amplifiers and optical fibers, an LiNbO.sub.3 optical modulator inserted in the loop is driven by an electric signal synchronized with a transmitted code to provide a gate on the time domain, retiming and waveform shaping of the transmitted signal are performed by the gate and optical pulses are propagated through the loop, thereby simulating a long distance optical fiber transmission.
According to Publication 1, however, since the LiNbO.sub.3 optical modulator is used which utilizes interference of light beams of specified wavelengths and specified directions of polarization, the gate waveform on the time domain is fixed and it is impossible to erase or remove accumulated noise having a spectral component of a wide time domain over which the gate is closed. Moreover, since the state of local polarization of the transmission system varies with time in the actual system, the modulation characteristic of the LiNbO.sub.3 optical modulator also varies with time, and hence the LiNbO.sub.3 optical modulator cannot be used.